A communication system as a successor of W-CDMA and HSDPA, namely, a Long Term Evolution (LTE) system has been considered by a W-CDMA standardization organization 3GPP. As a radio access scheme, Orthogonal Frequency Division Multiplexing (OFDM) is under consideration for downlink, and Single-Carrier Frequency Division Multiple Access (SC-FDMA) is under consideration for uplink (see 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA”, June 2006, for example).
In OFDM, a frequency band is divided into plural narrow frequency bands (sub-carriers), and data are placed on the respective divided frequency bands to carry out transmission. The sub-carriers are densely arranged in a frequency direction, allowing the sub-carriers to be partly overlapped without causing interference, thereby realizing high speed transmission and improving frequency usage efficiency.
In SC-FDMA, a frequency band is divided into plural narrow bands, and different narrow bands are used by different mobile stations, so that interference between the mobile stations can be reduced. According to SC-FDMA, which is characterized in that variations in the transmission power are reduced, a large coverage area and low energy consumption can be realized.
In LTE, one or more physical channels for both uplink and downlink are shared by plural mobile stations. The channel which is shared by plural mobile stations is typically called a shared channel. In LTE, a Physical Uplink Shared Channel (PUSCH) is used in uplink and a Physical Downlink Shared Channel (PDSCH) is used in downlink.
In uplink, a control channel (PUCCH: Physical Uplink Control Channel) and a random access channel (PRACH: Physical Random Access Channel) are used in addition to the shared channel. Furthermore, Demodulation Reference Signals (Demodulation RSs), Sounding Reference Signals (Sounding RSs), and so on are transmitted as pilot signals.
The PUCCH includes two types, i.e., a channel to be time-multiplexed with the PUSCH and a channel to be frequency-multiplexed with the PUSCH.
In the communication system which uses the shared channel, scheduling is needed to determine to which mobile station the shared channel is assigned for each subframe. Scheduling is performed based on information such as communication quality derived from the Sounding RSs.
In LTE uplink, the Sounding RSs are transmitted to mobile stations with various bandwidths in the whole system band. The Sounding RSs are time-multiplexed with the PUSCH.